1. Technical Field
The embodiments herein generally relate to channel estimation, and, more particularly to channel estimation for high Doppler mobile environments.
2. Description of the Related Art
High Doppler effects resulting from fast time varying dispersive channels represent the most critical impairment to channel estimation techniques in block transmissions. In multicarrier systems, it gives rise to the so-called intercarrier interference (ICI), whose modeling for correct data recovery is paramount. One of the current most challenging topics in wireless communications consists in the accurate modeling and implementation of channel estimation and symbol estimation methods for fast varying mobile channels. In such high Doppler environments, the channel variation within the transmitted block is so rapid, that the common notion of channel estimation no longer exists, and conventional linear estimation techniques do not apply.
In the case of orthogonal-frequency-division-multiplexing (OFDM) systems, the corresponding high Doppler frequency is translated into the so-called ICI, whose effect is to terminate the simplicity of estimation in cyclic prefix based schemes. One possible way to tackle the ICI problem is to capture, up to a certain extent, the channel variation within the OFDM block via a Taylor expansion of the exponential coefficients that correspond to a Jake's model approximation of a Rayleigh fading channel. The basic idea behind this approach is to consider the channel vector as a random quantity, in a way that all the channel derivatives can be cast into a linear model, suitable for estimation.
In approximate solutions that rely on both linear and decision directed estimation schemes, the approach leads to several open issues in terms of performance and feasibility of implementation, especially for digital video broadcasting (DVB) applications. The exact minimum mean-square-error (MMSE) channel parameters estimator possesses a matrix structure that becomes highly ill-conditioned, especially in DVB applications, exhibiting a condition number that grows with the ICI model order, and due to numerical problems, the structure of the estimator can lead to meaningless results.
Also, when estimating the channel parameters for the first time within a given OFDM block, training information is very limited. In this case, the type of receiver architecture (e.g., linear MMSE or decision-directed) can considerably affect the quality of the detected symbol, so that further channel and symbol estimations become compromised. Also, the algorithm employed in both channel and symbol estimation steps must have low complexity; i.e., it is usually desired that the underlying method makes use of the Discrete Fourier Transform (DFT) efficiency, or perhaps of the corresponding induced a Toeplitz or a circulant structure of the channel model. For instance, a MMSE receiver would require a matrix inversion whose complexity is prohibited. Preserving optimality and simultaneously implementing via a fast algorithm is a challenging task.